Pythonize input_type name in py_input_message (#436)

Co-authored-by: konstantin <konstantin.klein@hochfrequenz.de>
Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>
Fixes https://github.com/danielgtaylor/python-betterproto/issues/427
Fixes https://github.com/danielgtaylor/python-betterproto/issues/438

tests/inputs/service_uppercase/test_service.py

@@ -0,0 +1,8 @@
+import inspect
+
+from tests.output_betterproto.service_uppercase import TestStub
+
+
+def test_parameters():
+    sig = inspect.signature(TestStub.do_thing)
+    assert len(sig.parameters) == 5, "Expected 5 parameters"